<template>
    <div>
      <div class="box" v-bind:style="{backgroundColor:activeColor}" @click="changeBgColor"></div>
    </div>
</template>

<script>
export default {
  data(){
    return{
      activeColor:'red'
    }
  },
  methods:{
    changeBgColor(){
      this.activeColor='springgreen'
    }
  }
}
</script>

<style>
  .box{
    width: 100px;
    height: 100px;
    border:1px solid #000;
    border-radius: 50%;
  }
</style>